Capacitors are electrical devices that are used to accumulate and store electric charge. Capacitors are distinguished from batteries in at least two aspects. First, storage of electric charge in capacitors is based upon physical charge separation rather than the chemical separation of batteries. Second, charge and discharge rates of capacitors are much more rapid than the chemical reactions that occur in batteries.
In conventional capacitors, charge separation is maintained by two conductive plates that are separated by a dielectric material. In the presence of an applied potential, an electric field builds in the dielectric material and produces a mechanical force between the conductive plates. The ratio of the electric charge maintained on the conductive plates to the potential difference between them is referred to as the capacitance, which is measured in Farads.
Various modifications of conventional capacitors have also been developed. Electrolytic capacitors utilize an ion-containing liquid as one of its conductive plates. Such electrolytic capacitors typically display much higher capacitance values than do conventional capacitors. However, their utility is somewhat limited by a requirement that each conductive plate is to be maintained in a polarized voltage state.
Supercapacitors, also known as electric double-layer capacitors, electrochemical double-layer capacitors, supercondensors, ultracapacitors, or pseudocapacitors, can display even higher capacitance values. Supercapacitors differ significantly from conventional capacitors and electrolytic capacitors in that there is not a significant physical separation of the conductive plates in a supercapacitor. Instead, supercapacitors maintain charge separation by incorporating a vanishingly thin physical barrier between the conductive plates (<100 μm). The physical barrier effectively maintains charge separation when the supercapacitor is in the charged state, while being sufficiently permeable to charge carriers to allow rapid charge and discharge rates.
Many conventional supercapacitors presently use activated carbon particles as a high surface area substrate to hold charge carriers from an electrolyte dispersed therein. Although activated carbon particles have a high surface area, certain charge carriers are too large to penetrate the porous interior of the activated carbon particles and take advantage of its high surface area. Further, activated carbon is fairly non-compressible, and the volume of conventional supercapacitors containing a given quantity of activated carbon cannot typically be significantly reduced by compression.
FIG. 1 shows a schematic of an illustrative prior art supercapacitor 100 containing activated carbon particles 105. Supercapacitor 100 contains conductive layers 101 and 102, connected to positive terminal 103 and negative terminal 104, respectively. Conductive layers 101 and 102 each contain activated carbon particles 105 and an electrolyte containing positive ions 106 and negative ions 107 admixed with activated carbon particles 105. Positive ions 106 and negative ions 107 can reside about the interior or exterior of activated carbon particles 105. Conductive layers 101 and 102 are physically isolated from one another by a layer of separator material 108, which is permeable to positive ions 106 and negative ions 107 of the electrolyte. As shown in FIG. 1, supercapacitor 100 is in a discharged state.
Certain high performance materials, including carbon nanotubes, have been proposed as a replacement for activated carbon particles in supercapacitors due their high accessible surface area. Carbon nanotubes can be further advantageous in this regard due to their electrical conductivity. Although carbon nanotubes offer significant potential for improving the electrical performance of supercapacitors, research efforts to date have only been successful in randomly dispersing small quantities of carbon nanotubes in the electrolyte medium of a supercapacitor. As such, current fabrication techniques have only been amenable to production of small carbon nanotube-containing supercapacitors with low electrical storage capabilities.
In view of the foregoing, high-volume supercapacitors and other electrical devices containing large quantities of carbon nanotubes would represent a significant advance in the art. It would also be of considerable benefit to provide methods for readily preparing such high-volume supercapacitors and other electrical devices. The present invention satisfies these needs and provides related advantages as well.